Stimuli-responsive polymers represent one class of actuators that have the unique ability to change swelling behaviors, permeability, and elasticity in a reversible manner. Owing to these useful properties, stimuli-responsive polymers have numerous applications, particularly in medicine, pharmaceutics, drug-delivery, biosensors, separation and membranes, purifications, and enzyme and cell immobilization (Qiu, Y. and Park, K. Adv Drug Delivery Rev (2001), 53: 321-339; Miyata, T. Urgami, T. and Nakamae, K. Adv. Drug Delivery Rev. (2002), 54: 79-98; Bckiari, V. et al., Langmuir (2004), 20: 7972; Lopes, D. Cendoya, L. and Mijangos, C. Macromol Symp (2001), 166:173; Lao, L. and Ramanujan, V. R. J Mater Sci Mater Med (2004), 15: 1061; Porter, J. and Pickup, R. W. J. Microbiol. Methods (1998), 33: 221; Xie, X. et al., J Magn Mater (2004), 227: 16 and Gupta, P. K. and Hung, C. T. Life Sci (19889) 44, 175).
Magnetic gels, also called ferrogels, are a class of stimuli-responsive polymeric materials with their properties controlled by magnetic field. Ferrogels contain magnetic particles dispersed homogenously or heterogeneously and confined in a polymer network. Under a non-uniform magnetic field, particles undergo motion, which in turn induces elongation, contraction, or bending of the gels with short response time (Torok, G. et al., Phys B (2001) 297: 40; Szabo, D. Czako-Nagu, I. and Zrinyi, M. J Colloid Interface Sci (2002) 221: 166; Szabo, D. Szeghy, G. and Zrinyi, M. Macromolecules (1998) 31: 6541; Xulu, P. Filipesei, G. and Zrinyi, M. Macromolecules (2000) 33: 1716; Zrinyi, M. Feher, J. and Filipesei, G. Macromolecules (2000) 33: 5751; Zrinyi, M. Barsi, L. and Buki, A. J Chem Phys (1996) 104: 8750; and Zrinyi, M. Barsi, L. and Buki, A. Polym Gels Networks (1997) δ: 415). Although magnetic gels under go changes with short response time, these changes, especially changes in specific volume, are not substantial.
It would be desirable to have materials that can undergo substantial specific volume changes in response to an external stimuli.